Scanning electron microscopy

Transcription

1 Scanning electron microscopy Fei Quanta Tabletop Hitachi

2 Example: Tin soldier Pb M Sn L Secondary electrons Backscatter electrons EDS analysis Average composition

3 Learning goals: Understanding the principle Of the instrument Of type of signals detectors Understand how the parameters that you can/must change influences the picture and chemical analysis. Lab: Demonstration and hands on experience with our three different SEMs Table top SEM Environmental SEM High resolution SEM

4 Microscopes then and now

5 Limits to resolution Unaided eye ~ Light microscope ~ Scanning EM ~ Transmission EM ~ 0.1 mm 0.2 mm 1.0 nm 0.1 nm The higher the accelerating voltage, the smaller the wavelength of the electrons and the higher the possible achievable resolution.

6 Instrument: SEM is it like a TEM? In SEM, there are several electromagnetic lenses, including condenser lenses and one objective lens. Electromagnetic lenses are for electron probe formation, not for image formation directly, as in TEM. Two condenser lenses reduce the crossover diameter of the electron beam. The objective lens further reduces the cross-section of the electron beam and focuses the electron beam as probe on the specimen surface.

7 Objective lens The final probe-forming lens has to operate with a relative long working distance (WD= 10 mm), that is the distance between the specimen and lower pole-piece. This is necessary so that the emitted radiation can be collected and detected with desired efficiency. The long working distance increases the spherical aberration of the probeforming lens, which increases the size of the smallest attainable electron-beam spot. Objective lens Cross section Final Lens Probe Forming Labeled as Focus

8 Objective lens designs Out lens In lens Semi-in-lens Electron beam Electron beam Virtual lens Electron beam SED SED Virtual lens SED SED SED Specimen Specimen Virtual lens Features Features Features Easy to observe magnetic sample Possible to observe bigger sample Topographical imaging Deep depth of Focus Ultra high resolution High throughput observation at sample exchange position Variety of signal detecting system to optimize the contrast Specimen Ultra high resolution Possible to observe bigger sample at short WD Variety of signal detecting system to optimize the contrast

9 Compare the three different designs of objective lens : 1:Conventional lens Here the magnetic field of the lens is located inside the lens and the specimen sits away from the field. This leads to that the beam travels unprotected between lens and sample and is more vulnerable to EM disturbance. On the other hand the distance between lens and specimen only has limited effect on the signal strength. 2: Snorkel lens, semi-in-lens or immersion lens Here the magnetic field is projected down below the lens to enclose the specimen if it is located at short working distance. At the same time as the beam is protected from EM disturbance the electrons are effectively captured and led up through the lens to be detected by an in-lens detector. For longer working distance or when a side illumination effect is wanted, a classical SE detector is mounted in the chamber. 3: In-lens Here the sample sits on a TEM-type holder and has a maximum size of ca. 4x4x9 mm. Benefits here are excellent mechanical stability, high electron collection efficiency and very high EDS count rates.

10 Relationship between resolution and focal length

11 Different e - sources and gun types W LaB 6 Schottky FE Cold FE Source size 1 2 mm 1 2 mm nm 3-5 nm Temperature 2300 C 1500 C 1500 C Room temp Brightness [A/cm 2 sr] 1x10 6 1x10 7 5x10 8 2x10 9 Energy spread, E 2.0 ev 1.5 ev 0.5 ev 0.2 ev Stability,%/h 0.1 % 0.2 % 0.2 % 2-3 % Probecurrent 50 na -1 ma 50 na -1 ma >100 na na Life time 1 month 6 months 18 months > 5 years Gun vacuum 10-5 Torr 10-7 Torr 10-9 Torr Torr

12 Magnification of SEM is determined by the ratio of the linear size of the display screen to the linear size of the specimen area being scanned. The linear magnification is given by Electronbeam is scanned across the specimen and the procedure is known as Raster scanning. Raster scanning causes the beam to sequentially cover a rectangular area on the specimen. The signal electrons emitted from the specimen are collected by the detector, amplified and used to reconstruct the image according to one-to-one correlation between scanning points on the specimen and picture points on the screen of cathode ray tube (CRT). CRT converts the electronic signals to a visual display.

13 Condenser Lens Current and Resolution. The current in the condenser lens changes the spot size or diameter of the beam of electrons that scans the sample. More detailed information will be collected when the electron beam scans the same area with a smaller spot size. An increased current or a higher number for the condenser lens (CL) setting, will produce a smaller spot size and in general will result in a better resolution (A).

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15 Diatome Courtesy of Gokhan Taken by Quanta SEM microscope Magnification: 30000x Sample: Diatome Detector: ETD Voltage: 4.0kV Vacuum: 4.01e-4Pa Horizontal Field Width: 9.93μm Working Distance: 7.7 Spot: Same picture - different size Magnification?

16 Beam samples interaction:

17 Scattering Inelastic scattering refers to a variety of physical processes that act to progressively reduce the energy of the beam electron by transferring that energy to the specimen atoms through interactions with tightly bound inner-shell atomic electrons and loosely bound valence electrons. Although the various inelastic scattering energy loss processes are discrete and independent, Bethe (1930) was able to summarize their collective effects into a continuous energy where E is the beam energy (kev), Z is the atomic number, ρ is the density (g/cm3), A is the atomic weight (g/mol), and J is the mean ionization potential (kev) given by

18 Beam interaction volumes Sample = Si 1μm Vacc : 10kV Vacc : 1kV Images are formed because of beam interaction with the sample This happens in a volume, not in a point The size of this volume varies with beam energy...

19 Beam Excitation Volumes

20 Elastic scattering Simultaneously with inelastic scattering, elastic scattering events occur when the beam electron is deflected by the electrical field of an atom (the positive nuclear charge as partially shielded by the negative charge of the atom s orbital electrons), causing the beam electron to deviate from its previous path onto a new trajectory, as illustrated schematically in the figure. The elastic scattering crossection, can be used to estimate how far the beam electron must travel on average to experience an elastic scattering event, a distance called the mean free path, λ The mean free path is of the order of nm. Elastic scattering is thus likely to occur hundreds to thousands of times along a Bethe range of several hundred to several thousand nanometers.

21 Accelerating voltage

22 Signals in the SEM SE Inner information Electron beam 一一 High energy BSE (I) 一 Low energy BSE (II) 一 一 SE II 一 一 一 一 一 一 SE I SE Surface information Z 一 一 sample SE 一 一 一 一 一 SE escape depth BSE escape depth Z

23 Electron amount Emitted electrons (SE) (BSE) Secondary electron Back scattered electron 50eV Energy of signal electron (ev) (Notice that the scale is logarithmic) When a sample is hit by an electron beam a variety of electron emissions are available.

24 Huskelapp The group of secondary electrons (SE) can be divided into 4 groups. There are SEI, SEII, SEIII, and SEIV. These electrons are classed according to how they are generated. A SEI is an electron is an electron that is generated at the point of primary beam impingement in surface of the specimen. Thus, it carries the highest resolution information. A SEII is an electron that is generated when a backscattered electron leaves the surface of the specimen. Due to the energy of backscattered electrons, this SEII could leave the surface of the specimen microns away from the primary beam impingement site. SEIIs hurt the resolution of the image, but add greatly to overall image brightness. A SEIII is an electron released when an energetic backscattered electron strikes the interior of the specimen chamber, causing a SE to be released. SEIVs are formed when the primary beam strikes an aperature within the electron column. SEIIIs and SEIVs contribute noise to the image. By understanding signal formation, the specimen can be properly prepared for analysis.

25 SE yield variation The rapid change in the incident electron beam range causes a large, characteristic variation in the SE yield Typically the yield rises from ~0.1 at 30keV to in excess of 1 at around 1keV, and as high as 100 for some materials Experimental SE yield data for Ag

26 Why the SE yield changes SE escape depth is ~ 3-5nm At high energies most SE are produced too deep to escape so the SE yield d is low But at lower energies the incident range is so small that most of the SE generated can escape so the SE yield rises rapidly At very low energies fewer SE are produced because less energy is available so the SE yield falls again high voltage low voltage interaction volumes

27 Do high and low kv SE images look the same? compared to the high energy The image looks less 3-D Highlighting is absent Surface junk is more visible

28 Emitted electrons SE:s Change in SE yield at different incident e-beam angles. Occurrence of edge effect in fine structured surfaces. 28

29 Resolution At high energy the SE1 signal typically comes from a volume 3-5nm in diameter, but the SE2 signal from a volume of 1-3µm in diameter. But at low energy the SE1 and SE2 electrons emerge from the same volume because of the reduction in the size of the interaction volume

30 Resolution at ultra-low energies 30eV 500eV 100nm Because C s and C c decrease with the landing energy the imaging resolution is only limited by diffraction

31 BSE Backscatter electrons SE Inner information Electron beam 一一 High energy BSE (I) 一 Low energy BSE (II) 一 一 SE II 一 一 一 一 一 一 SE I SE Surface information Z 一 一 sample SE 一 一 一 一 一 SE escape depth BSE escape depth Z

32 BSE vs. SE detection BSE detector SE Sample : Polyvinyl Alcohol Accelerating Voltage : 3kV, Vacuum: 60Pa, Mag. : 1,000x Sample : Polyvinyl Alcohol Accelerating Voltage : 3kV, Vacuum: 60Pa, Mag. : 1,000x

33 Emitted electrons BSE:s The count ratio of BSE:s depends on the mean atomic number of the specimen. 33

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35 Detectors on the SEM Electron Detectors: Everhart-Thornley (E-T) detector Located on one side and has a small solid angle of detection.

36 Detectors on the SEM Electron Detectors: Solid-State Diode detector Located on pole piece and has a large solid angle of detection Electron-hole pairs are produced by the action of high energy backscattered electrons. Annular detector split into two semi-circles, A and B. A+B: compositional mode A-B: topographic mode

37 SU8000Series standard optics Variety of signal detection system in SU8200-series Top Upper Signal type Signal name Detector information BSE HA- BSE Top Composition, crystal BSE LA- BSE Upper Composition + Topo (Charge suppression) Electrode SE SE Upper Surface information (Including voltage contrast) Lower SE Lower Lower Topo STEM STEM BF- 1 STEM DF- 1 STEM STEM Lower Sample internal information + Crystal Sample internal information + Composition SE STEM Detector BSE 37 STEM

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39 Lower detector Electrode LOWER(SE-L) Top EXB Upper Lower Sample SE BSE Sample Vacc : 3.0V Mag. : Photocatalyst : x 50k Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato courtesy of : Model :SU8020 Topographical image with shadow

40 Upper detector: Pure SE Electrode UPPER(SE) Top EXB Upper Lower Sample Control electrode SE BSE Nagaoka University of Technology, Faculty of Engineering, Sample : Photocatalyst Dr. Kazunori Sato courtesy of : Vacc : 3.0V Mag. : x Surface 50k Model information :SU8020 (incl. Voltage contrast)

41 Upper detector: SE Filtering, LA-BSE signal Electrode UPPER(LA-BSE) Top EXB Upper Lower Sample Control electrode SE BSE Sample Vacc : 3.0V Mag. : Photocatalyst : x 50k Nagaoka University of Technology, Faculty of Engineering, Dr. Kazunori Sato courtesy of : Model :SU8020 Topographical + Compositional information

42 Top detector: High-angle BSE Electrode TOP(HA-BSE) Top EXB Upper Lower SE BSE Sample Control electrode Nagaoka University of Technology, Faculty of Engineering, Sample : Photocatalyst Dr. Kazunori Sato courtesy of : Vacc : 3.0V Mag. : x Compositional 50k Model + Crystal :SU8020 information (Less topographical information)

43 LOWER (SE-L) UPPER (SE) TOP (HA-BSE) UPPER (LA-BSE)

44 Deceleration (cathode lens) In deceleration, a negative voltage is applied to the specimen to decelerate primary electrons before arriving at specimen surface. Primary beam Objective lens Normal mode V R = 0 V Vi = 0.5kV Vi = V o - V R Landing voltage: Vi -V R Specimen -V R : Deceleration voltage Deceleration V R = 1.5kV Vi = 0.5kV Optimum αi Expands Improved resolution

45 But if we accelerate the SE:s? Top Upper Top Upper : Low energy signal : High energy signal V d SE Accelerated SE Accelerated BSE 1 Both of the SE and BSE are accelerated by the deceleration voltage(negative bias) 2 Makes it more difficult to only select the low energy electrons 3 Sample bias and the accelerated BSE:s increase the signal and contrast

46 Lecture 2

47 Microscopy Lecture II EDS, EDX Electron Beam Characteristic X-rays Secondary Electrons (SE) Backscattered Electrons (BSE) Cathodoluminescence Heat Auger Electrons Absorbed Electrons Transmitted Electrons

48 Parameters, imaging, conclusion? We can change 1. Accelerating voltage 2. Magnification 3. Spot size 4. Working distance 5. Beam current 6. Detectors Ask questions in lab: We want: High resolution We have choosen low Accelerating voltage We will repeat inlens detectors Next week

49 Plan: X-rays Detectors Qualitative analyses Quantitative analyses Data from lab(?) Rapport

50 SE and BSE

51 Average PbM Sn L

52 Atomic energy levels and line transition Transitions have different probabilities Lines have different intensities

53 EDS spectra: Origin of Bremsstrahlung and characteristic peaks 1 kev = J

54 EDS spectra: Origin of Bremsstrahlung and characteristic peaks continuum or Bremsstrahlung (breaking radiation) results from deceleration of beam electrons in the electromagnetic field of the atom core combined with energy loss and creation of an X-ray with the same energy

55 Hmmm. Observerte data fra SEM + EDS

56 EDS spectra: Origin of Bremsstrahlung and characteristic peaks cps/ev - Characteristic X-rays are formed by excitation of inner shell electrons - Inner shell electron is ejected and an outer shell electron replaces it - Energy difference is released as an X-ray Fe Mn Cr Mn C Ni Si Cr Fe Ni kev

57 EDS spectra: Origin of Bremsstrahlung and characteristic peaks If beam energy E > E K then a K-electron may be excited Energy of emitted photon can be calculated: E Phot = E 1 E 2 e.g.: Fe L K E 1 = E K = 7.11 kev E 2 = E L = X-ray energy is the difference between two energy levels! 0.71 kev E Ka = 6.40 kev 0.71 kev 7.11 kev 6.40 kev

58 X-ray and AUGER generation process Emission of X-ray Emission of Auger electron Auger and X-ray yield are competing processes

59 Fluorescence yield (ω) ω= fraction of ionisation events producing characteristic X-rays (rest produce Auger electrons) Ge C - ω increases with Z - ω for each shell: ωk ωl ωm - Auger process is favoured for low Z, - fluorescence dominates for high Z + A = for C K 0.5 for Ge K

60 Characteristic peaks: K, L, M series Kα L-familie Fe K-familie Kβ Energy of characteristic peaks is defined by element The higher the atomic number Z the higher the peak energy

61 The K-family of lines S (16) Ca (20) Mn (26) K 1, ev 3692 ev 5900 ev K 2464 ev 4013 ev 6492 ev (K - K 1,2 ) 156 ev 319 ev 592 ev

62 The L-family of lines Mo (42) Ce (58) L ev 4839 ev L 2394 ev 5262 ev L l 2014 ev 4287 ev L l

63 Line intensity relations (2) Lα 1 Lβ 1 Lβ 2 Lγ 1 Ll Lα 2 Lγ 2/3 Spectrum Barium L series, 15 kv α1 : β1 : γ1 = 100 : 52 : 10

64 Line intensity relations (3) Lα 1 (100) Ti-Kα1 BaTiO 3 Barium Ll Lα 2 Lβ 1 (31) Ti-Kβ1 Lβ 2 Lγ 1 (5) Lγ Lγ 2/3 2/3 Spectrum Barium L-series, 15 kv α1 : β1 : γ1 = 100 : 52 : 10 Spectrum BaTiO 3, 15 kv Overlapped Ba L-series and Ti K-series

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66 Intensity and energy of characteristic lines - Energy of line is defined by - Element - Type of transition - Intensity of line is defined by - probability of producing a hole (vacancy) - probability of electron transition - probability of x-ray emission - concentration

67 Probability of producing a hole: Ionization cross-section for electrons Ionisation cross section for electrons Ionization cross section: probability of excitation maximum ionization cross section: 2,5 x E bind Important for the selection of accelerating voltage.

68 Ionization cross section for electrons A sample with equal amount of Cr,Fe and Ni. Cr: 33% Cr Fe Ni Fe: 33% Ni: 33% U = 10 kev E Cr Fe Ni E exc bind : 1,847 1,561 1,337

69 30 kv and 10 kv Fe/Cr=0,8 Ni/Cr=0,7 Fe/Cr=0,5 Ni/Cr=0,15

70 Isolated Atoms When isolated atoms are considered, the probability of an energetic electron with energy E (kev) ionizing an atom by ejecting an atomic electron bound with ionization energy Ec (kev) can be expressed as a cross section, QI: where ns is the number of electrons in the shell or subshell (e.g., nk = 2), and bs and cs are constants for a given shell (e.g.,bk = 0.35 and ck = 1) E= beam energy Ec= ionization energy

71 5.) X-ray range electron beam (E 0 ) sample surface secondary electrons ca µm ca. 10 µm 3 back-scattered electrons X-rays bremsstrahlung Different excitation ranges for: Bremsstrahlung, - characteristic x-ray radiation and - secondary electrons (SE) - back-scattered electrons (BSE)

72 Detectors on the SEM X-ray Detectors EDS Spectrometer WDS Spectrometer MEF4010 Scanning electron microscopy

73 X-RAY-detectors WDX wavelength dispersive EDX Energy dispersive Energy resolution is 100 times better with the WDX ev EDX 10 ev WDX

74 Kvalitativ analyse

75 Hva har vi tenkt på Valg av akselerasjonsspenning Utsnitt på prøven Vinkel tilting av prøven Arbeidsavstand (WD) Telletid Oppladning (?) Mål: Gode data med god tellestatistikk og god oppløsning

76 Vi kan finne ut mer

77 Quantitative

78 5.) X-ray range Monte Carlo electron-trajectory simulations of interaction volume in iron as function of primary beam energy EHT = 10 kv EHT = 20 kv EHT = 30 kv R d 0,4 µm R d 1,3 µm R d 2,5 µm With higher primary electron energy penetration depth is increasing

79 5.) X-ray range Monte Carlo electron-trajectory simulations of interaction volume as function of atomic number (EHT = 15 kv) Carbon R d 2 µm Iron R d 0,6 µm Gold R d 0,2 µm With higher density penetration depth is decreasing

80 The kv compromise I char increases with increasing E 0 /E c X-ray signal improves R x increases with increasing E 0 /E c X-ray spatial resolution degrades E 0 U ,5 EC

81 Optimum overvoltage The ionization cross-section describes the probability that a particular event will take place: For X-rays: Q = 6.5x10-20 n s b s UE c ln(c s U) n s = number of electrons in a shell E c = critical excitation voltage b s & c s = constants related to the electron shell U = overvoltage (E o /E c ) QE (cm 2 kev 2 ) U = E o /E c Optimum overvoltage U is around Without sufficient overvoltage, x-ray production is dramatically lowered. U

82 ZAF

83 Can't calculate ZAFs unless concentration is known. But we don t know the concentration?? Use k values (I/I = k) to estimate compositions of each element. Then calculate ZAFs, and refine by iteration Z at. no. correction Function of electron backscattering factor & electron stopping power - depend upon the average atomic number of unknown and standard Dependent on ionization cross section Varies with composition and accelerating voltage A absorption correction Varies with m, takeoff angle, accelerating voltage F fluorescence correction primary fluorescent x-rays > secondary fluorescent x-rays Varies with composition and accelerating voltage

84 CalcZAF

85 kv=15 vinkel = 40

86 Mass absorption coefficients Mass absorption coefficients are stored as a matrix of numbers of absorption of a particular X-ray line (the emitter) by an absorber: For example, a portion of the MAC matrix for Kα X-rays for Z = 23 to 29 is shown below. Mass absorption coefficients Emitter V 4952 ev Cr 5415 ev Mn 5899 ev Fe 6403 ev Co 6930 ev Ni 7478 ev Cu 8048 ev V Cr Mn Fe Co Ni Cu Note that the MAC for absorption of Fe Kα by Co (79.0) is different from the MAC for absorption of Co Kα by Fe (57.1). ds/quantitative/

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88 Sample surface and absorption electron beam SDD rough surface X-rays tilted polished surface take-off angle polished surface negative tilt angle -TA d2 d1 d1: distance to (imaginary) polished surface d2: actual distance to specimen surface